Climates of the Past

Our climate has been constantly changing since Earth began, with periods of global warming and global cooling long before human beings and their activities began. Modern weather measurements go back only 100 to 150 years. So how do we know what Earth and its climate were like in prehistoric times?

Some clues come from Earth itself. Natural recorders of climate variations are called proxies; they include ocean and lake sediments, ice cores, fossils, tree rings, and corals. Through proxies, scientists can deduce long-term regional temperature trends as well as changes in the atmosphere's chemical makeup. For example:

  • Tree rings show what climate factors shaped each year of a tree’s life.
  • Bubbles trapped inside ice fields hold air from many thousands of years ago.
  • Tiny ocean creatures called foraminifera reflect the state of temperatures and nutrients.
  • Shells and other debris deposited in layers of sediment on the bottom of lakes and oceans can be analyzed to provide insights into past climate, year by year.

We also know that the orbits of the Sun and Earth undergo a variety of cyclic shifts over thousands and millions of years. Tiny changes in the tilt of Earth and the asymmetry of its path around the Sun can make a big difference to regional climate—sometimes enough to trigger an ice age.

A thousand years of climate

With these clues in hand, NCAR scientists peer into our climate history using numerical models run on large supercomputers. These complex, sophisticated software packages can trace the flow of climate over hundreds and even thousands of years. They rely on our basic understanding of atmospheric physics and chemistry, as well as the clues gathered from Earth itself.

Paleoclimate models can look even further back, reproducing slices of atmospheric time from millions of years ago. One NCAR study of the climate 251 million years ago lends support to the notion that a sudden increase in carbon dioxide helped trigger the greatest mass extinction in Earth’s history.

Scientific and public debate has swirled around overall portraits of temperature increase like the one shown here. These are often dubbed "hockey stick" graphs because of the sharp rise on the right-hand side. Recent research at NCAR and elsewhere has bolstered the case for this representation of a dramatic temperature increase over the last century.

Critics have questioned the statistical methods used to convert temperature proxies into global averages for the period prior to the late 1800s (when modern observations began). A number of recent studies have re-examined the methods used and suggested adjustments. Yet the basic message of the hockey stick remains valid, as a panel of the National Academy of Sciences found in a 2006 report. That panel concluded that the warming trend since 1900, and especially since the 1970s, is highly unusual and perhaps unprecedented in the last millennium.

Research groups at NCAR and other centers use data from climate proxies like tree rings and air bubbles trapped in ice cores to reconstruct the ups and downs in global average temperature. Research groups at NCAR and other centers use data from climate proxies like tree rings and air bubbles trapped in ice cores to reconstruct the ups and downs in global average temperature. This graphic from a 2006 report by a panel of the National Academy of Sciences shows seven different reconstructions of the past 1100 years of climate, from 900 to 2000. Each of these studies used different methods and a different set of proxies collected from a different part of the Northern Hemisphere.

The black line starting in the late 1800s shows actual temperature measurements, known as the instrumental record. All the other colors show reconstructions based on individual proxies (such as tree rings or ice cores from different ice caps), or on combinations of proxies. The multiproxy studies take advantage of the strengths of the separate proxies while minimizing the influence of limitations associated with the individual records.

The gray background gives a qualitative idea of how uncertainty increases further back in time. The darker the background, the greater the uncertainty regarding the temperature signal emerging from a reduced number of samples.

Temperatures by the mid-20th century were at or above the levels seen during the "Medieval Warm Period," near the start of the graph in 1000 C.E. Since the 1970s, the global average has risen more than 0.4°C (0.7°F). (Image courtesy National Research Council, National Academy of Sciences.)